Lecture 3－Formatted Input/Output

Meng-Hsun TsaiCSIE, NCKU

scanf() printf()

The printf Function

• The printf function must be supplied with a format string, followed by any values that are to be inserted into the string during printing:

printf(string, expr1, expr2, …);

• The format string may contain both ordinary characters and conversion specifications, which begin with the % character.

• A conversion specification is a placeholder representing a value to be filled in during printing.

• %d is used for int values

• %f is used for float values

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The printf Function (cont.)

• Ordinary characters in a format string are printed as they appear in the string; conversion specifications are replaced.

• Example:int i, j;

float x, y;

i = 10;

j = 20;

x = 43.2892f;

y = 5527.0f;

printf("i = %d, j = %d, x = %f, y = %f\n", i, j, x, y);

• Output:i = 10, j = 20, x = 43.289200, y = 5527.000000

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The printf Function (cont.)

• Compilers aren’t required to check that the number of

conversion specifications in a format string matches the

number of output items.

• Too many conversion specifications:printf("%d %d\n", i); /*** WRONG ***/

• Too few conversion specifications:printf("%d\n", i, j); /*** WRONG ***/

• Compilers aren’t required to check that a conversion

specification is appropriate.

• If the programmer uses an incorrect specification, the

program will produce meaningless output:printf("%f %d\n", i, x); /*** WRONG ***/

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Conversion Specifications

• A conversion specification can have the form %m.pX or %-m.pX, where m and p are integer constants and X is a letter.

• Both m and p are optional; if p is omitted, the period that separates m and p is also dropped.

• In the conversion specification %10.2f, m is 10, p is 2, and X is f.

• In the specification %10f, m is 10 and p (along with the period) is missing, but in the specification %.2f, p is 2 and m is missing.

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Minimum Field Width m

• The minimum field width, m, specifies the minimum number of characters to print.

• If the value to be printed requires fewer than m characters, it is right-justified within the field.

• %4d displays the number 123 as •123. (• represents the space character.)

• If the value to be printed requires more than m characters, the field width automatically expands to the necessary size.

• Putting a minus sign in front of m causes left justification.

• The specification %-4d would display 123 as 123•.

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12345 12345%4d

Precision p on %d

• The meaning of the precision, p, depends on the choice of X, the conversion specifier.

• The d specifier is used to display an integer in decimal form.

• p indicates the minimum number of digits to display (extra zeros are added to the beginning of the number if necessary).

• If p is omitted, it is assumed to be 1.

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%.3d -> 001

Precision p on floating-point %e, %f and %g

• Conversion specifiers for floating-point numbers:

e — Exponential format. p indicates how many (rounded) digits should appear after the decimal point (the default is 6). If p is 0, no decimal point is displayed.

f — “Fixed decimal” format. p has the same meaning as for the e specifier.

g — Either exponential format or fixed decimal format, depending on the number’s size. p indicates the maximum number of significant digits to be displayed. The g conversion won’t show trailing zeros. If the number has no digits after the decimal point, g doesn’t display the decimal point.

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8.392e+02

839.210

float x = 839.21;

839

%.3e

%.3f

%.3g

Program: Using printf to Format Numbers

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#include <stdio.h>

int main(void)

{

int i;

float x;

i = 40;

x = 839.21f;

printf("|%d|%5d|%-5d|%5.3d|\n", i, i, i, i);

printf("|%10.3f|%10.3e|%-10g|\n", x, x, x);

return 0;

}

|40| 40|40 | 040|

| 839.210| 8.392e+02|839.21 |

tprintf.c

Escape Sequences

• The \n code that used in format strings is called an escape sequence.

• Escape sequences enable strings to contain nonprinting (control) characters and characters that have a special meaning (such as ").

• A partial list of escape sequences:

Alert (bell) \a

Backspace \b

New line \n

Horizontal tab \t10

“NTU\b\bCKU” “NCKU”

Escape Sequences (cont.)

• A string may contain any number of escape sequences:

printf("Item\tUnit\tPurchase\n\tPrice\tDate\n");

• Executing this statement prints a two-line heading:

Item Unit Purchase

Price Date

• Another common escape sequence is \", which represents the "character:

printf("\"Hello!\"");/* prints "Hello!" */

• To print a single \ character, put two \ characters in the string:

printf("\\"); /* prints one \ character */

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The scanf Function

• A scanf format string may contain both ordinary charactersand conversion specifications.

• The conversions allowed with scanf are essentially the same as those used with printf.

• When using scanf, the programmer must check that the number of conversion specifications matches the number of input variables and that each conversion is appropriate for the corresponding variable.

• The & symbol, which normally precedes each variable, is usually (but not always) required, and it’s the programmer’s responsibility to remember to use it.

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The scanf Function (cont.)

• In many cases, a scanf format string will contain only conversion specifications:

int i, j;

float x, y;

scanf("%d%d%f%f", &i, &j, &x, &y);

• Sample input:

1 -20 .3 -4.0e3

scanf will assign 1, –20, 0.3, and –4000.0 to i, j, x, and y, respectively.

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How scanf Works

• scanf tries to match groups of input characters with conversion specifications in the format string.

• For each conversion specification, scanf tries to locate an item of the appropriate type in the input data, skipping blank space if necessary.

• scanf then reads the item, stopping when it reaches a character that can’t belong to the item.

• If the item was read successfully, scanf continues processing the rest of the format string.

• If not, scanf returns immediately.

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How scanf Works (cont.)

• As it searches for a number, scanf ignores white-space characters (space, horizontal and vertical tab, form-feed, and new-line).

• A call of scanf that reads four numbers:scanf("%d%d%f%f", &i, &j, &x, &y);

• The numbers can be on one line or spread over several lines:1

-20 .3-4.0e3

• scanf sees a stream of characters (¤ represents new-line):••1¤-20•••.3¤•••-4.0e3¤ssrsrrrsssrrssssrrrrrr (s = skipped; r = read)

• scanf “peeks” at the final new-line without reading it.

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How scanf Works (cont.)

• When asked to read an integer, scanf first searches for a digit, a plus sign, or a minus sign; it then reads digits until it reaches a nondigit.

• When asked to read a floating-point number, scanf looks for• a plus or minus sign (optional), followed by• digits (possibly containing a decimal point), followed by• an exponent (optional). An exponent consists of the letter e

(or E), an optional sign, and one or more digits.

• %e, %f, and %g are interchangeable when used with scanf.

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How scanf Works (cont.)

• When scanf encounters a character that can’t be part of the current

item, the character is “put back” to input buffer (to be read again later).

• Sample input:

1-20.3-4.0e3¤

• The call of scanf is the same as before:

scanf("%d%d%f%f", &i, &j, &x, &y);

• Here’s how scanf would process the new input:• %d. Stores 1 into i and puts the - character back.• %d. Stores –20 into j and puts the . character back.• %f. Stores 0.3 into x and puts the - character back. • %f. Stores –4.0 × 103 into y and puts the new-line character

back.

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Ordinary Characters in Format Strings

• When it encounters one or more white-space characters in a format string, scanf reads white-space characters from the input until it reaches a non-white-space character (which is “put back”).

• When it encounters a non-white-space character in a format string, scanf compares it with the next input character.

• If they match, scanf discards the input character and continues processing the format string.

• If they don’t match, scanf puts the offending character back into the input, then aborts.

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"%d %d"

"%d/%d"

Ordinary Characters in Format Strings (cont.)

• Examples:

• If the format string is "%d/%d" and the input is •5/•96, scanf succeeds.

• If the input is •5•/•96 , scanf fails, because the /in the format string doesn’t match the space in the input.

• To allow spaces after the first number, use the format string "%d /%d" instead.

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Confusing printf with scanf

• Although calls of scanf and printf may appear similar, there are significant differences between the two.

• One common mistake is to put & in front of variables in a call of printf:

printf("%d %d\n", &i, &j); /*** WRONG ***/

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Confusing printf with scanf (cont.)

• Consider the following call of scanf:

scanf("%d, %d", &i, &j);

• scanf will first look for an integer in the input, which it stores in the variable i.

• scanf will then try to match a comma with the next input character.

• If the next input character is a space, not a comma, scanf will terminate without reading a value for j.

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5 8

Confusing printf with scanf (cont.)

• Putting a new-line character at the end of a scanfformat string is usually a bad idea.

• To scanf, a new-line character in a format string is equivalent to a space; both cause scanf to advance to the next non-white-space character.

• If the format string is "%d\n", scanf will skip white space, read an integer, then skip to the next non-white-space character.

• A format string like this can cause an interactive program to “hang.”

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Program: Adding Fractions

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/* Adds two fractions */#include <stdio.h>int main(void)

{

int num1, denom1, num2, denom2, result_num, result_denom;

printf("Enter first fraction: ");

scanf("%d/%d", &num1, &denom1);

printf("Enter second fraction: ");

scanf("%d/%d", &num2, &denom2);

result_num = num1 * denom2 + num2 *denom1;

result_denom = denom1 * denom2;

printf("The sum is %d/%d\n",result_num, result_denom);

return 0;

}

addfrac.c

Enter first fraction: 5/6

Enter second fraction: 3/4

The sum is 38/24

A Quick Review to This Lecture

• The printf() function

• Format string

• ordinary character (abcd)

• conversion specifications (%d, %f, %e, %g)

• Conversion specification for X: %m.pX or %-m.pX (right or leftjustification)

• m: minimum field width

• p: precision

• Escape Sequences ( \n \t \” \\ \a \b )

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printf(string, expr1, expr2, …);

A Quick Review to This Lecture (cont.)

• The scanf() function

scanf("%d%d%f%f", &i, &j, &x, &y);

• Format string similar to that for printf()

• %e, %f, and %g are interchangeable with scanf.

• In most cases, put & symbol in front of each variable

• Ordinary characters in format string

• White-space: read white spaces until reaching non-white-space

• Non-white-space:

• Match: discard char. and continue reading

• Not match: put back char. and abort reading

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"%d %d"

"%d/%d"

A Quick Review to This Lecture (cont.)

• Confusing scanf() and printf()

• Put & in front of variables in printf:

printf("%d %d\n", &i, &j);

• Put unnecessary char. in format string in scanf:

scanf("%d, %d", &i, &j);

• Put a new-line char. at the end of a scanf format string :

scanf("%d\n", &i);

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may cause an interactive program to “hang.”

may terminate without reading the unnecessary char.

will print out unexpected value